1. Field of Invention
The present invention relates generally to the field of oil and gas exploration and specifically to the interpretation of seismic data for oil and gas exploration. Specifically, this invention provides an improved method for automatically extending interpreter-selected horizons in seismic data.
2. Background Art of the Invention
Seismic interpretation is the science and art of deducing geologic history by delineating geological surfaces as represented in and by seismic data. Sand and mud (which over time harden and become rocks) are naturally deposited in nearly horizontal layers. Boundaries between layers of buried rock are used to reveal subsurface geometry. The boundaries between layers are known as “geologic surfaces”, and rocks between boundaries are commonly known as “formations”.
As a general definition, “geologic surfaces” are boundaries between rocks when seen at the surface, or as they would be seen if at the surface, or as seen in wells. Although geologic surfaces do not appear in seismic data, they are deduced from seismic “horizons”. Horizons are interpreted from seismic features that are aligned approximately horizontally and represent the geological structures in the seismic data.
Some rocks in the subsurface are porous, similar to beach sand. Porous subsurface rocks contain a mixture of water, oil, and/or gas. Oil and gas are lighter than water and tend to separate and float upward. The path of movement and the cessation of movement is in large part dependent on the geometry of the rocks in which the fluids move, and this makes the geometry of interest to seismic interpreters.
Knowledge of rock geometry is also important for understanding the history of the rocks. Layer geometry may reveal the presence of buried river channels or beaches (which are more likely to be porous). If layers are tilted, bent, or broken, the geometry may reveal that rocks moved after they were buried. Seismic data provide a relatively inexpensive way to discover subsurface geometry. Layer history can be important in more sophisticated analyses regarding the generation of oil and gas and the re-distribution of porosity over time.
Geologic “facies” is a key concept in interpretation. A “facies” is a combination of rock type and geologic setting. Sand bars in a river channel are an example of a geologic facies, as are sediments that breach channel boundaries.
As stated above, horizons (not geologic surfaces) are seen in seismic data. Seismic data are acquired by transmitting acoustic signals (generated by dynamite, for example) into the Earth and recording the echoes. Echoes are caused by changes in the acoustic properties of rock from layer to layer, just as echoes in a canyon are caused by the difference in acoustic properties between air and rock. If echoes are laterally consistent, the pattern is understood to reveal a “surface of reflection”.
Seismic data are acquired digitally on land, on sea, and on the sea floor. Data are transmitted to processing centers, where noise is filtered out and the data are otherwise conditioned for interpretation. From the processor, seismic data are loaded onto “computer workstations” (a term that includes personal computers as well as more powerful graphic hardware, specialized processors, and general purpose computers) for interpretation.
FIG. 1A depicts a typical record of seismic echoes as detected by a single receiver at the surface. It is a sinusoidal curve as a function of time. Seismic echoes oscillate between compression and rarefaction over a period of several seconds, and this rise and fall in pressure with time is recorded for processing and analysis.
One of the challenges of seismic interpretation is interference between layers. Just as echoes in a canyon can become garbled, a degree of confusion can be mixed with seismic signals. This is the primary reason that horizons cannot be taken at face value as geologic surfaces.
A single recorded echo, as shown in FIG. 1A, is called a “seismic trace.” An interpreter may have several thousand or several million traces to interpret. FIG. 1B shows an enhancement of the same trace as shown in FIG. 1A. Where the signal moves to the right above zero (into compression), the line marking the signal has been filled in with black. The configuration of a trace reflecting to the right and then returning toward zero, as highlighted in black, is called a “peak” in the trace. Excursion to the left with a return to zero is called a “trough”.
To detect geometric relationships, echoes must be collected along a line or over an area, and seismic receivers are typically laid out along a line or in a grid pattern over the surface of the Earth. Each receiver, in effect, affords a “peephole” into the subsurface, and geometry is detected by examining data from many such adjacent peepholes.
Even when collected in grids, seismic data are typically displayed as single lines for interpretation. These vertical cross sections, present a line of collected data or one line in a grid of collected data. A line display provides a profile view of the seismic echoes so that one can readily see differences in the echoes vertically and laterally and along the line.
FIG. 2A depicts a typical seismic line as a collection of contiguous traces, each trace as shown in FIG. 1A. FIG. 2B shows the same seismic line as typically displayed for interpretation, using the display technique of FIG. 1B for each trace. Traces are typically 50 to 150 feet apart.
One aspect of interpretation is the mechanical marking of surfaces deemed important by the interpreter. Marking these surfaces is done by interpreters on computer workstations when they electronically “draw” lines on a seismic section as displayed by a workstation. Each drawn line represents an interpreted horizon at that location. An interpretation project will typically generate several dozen and sometimes hundreds of horizons. If the seismic data were collected on a grid, each horizon is likely to be found on numerous sections.
Horizons may be displayed in color so that they can be distinguished from one another and from the seismic data itself. FIG. 2C is the same as FIG. 2B with the addition of a typical interpreted horizon, as indicated by the arrow. In this interpretation, the interpreter has followed seismic peaks across the section.
To alleviate the tedium of picking dozens of horizons over hundreds of seismic sections, most interpretation software applications provide “autopickers” or “autotrackers” that compute picks automatically based on a starting set of picks. As a result, only a small percent of the total picks are picked manually.
In general, autopickers proceed in a calculated direction from one picked location to the next. Typically, the direction is horizontally from the starting trace to surrounding unpicked traces. Possible picks on surrounding traces are scored, and either the one or several with the best score(s) is/are retained as picks.
U.S. Pat. No. 5,056,066 (Howard, 1991) describes the typical process of autopicking, whereby picks are selected laterally from trace to trace, starting with the seed pick. Most autopickers start with this conceptual and mechanical framework. Differences between methods are in the particular way picks are selected. U.S. Pat. No. 5,056,066 compares several traces in the area with the goal of obtaining a more reliable pick.
U.S. Pat. No. 5,153,858 (Hildebrand, 1992) and U.S. Pat. No. 5,251,184 (Hildebrand et al, 1993) address the speed and efficiency of picking, while U.S. Pat. No. 5,537,365 (Sitoh, 1996) scores the sufficiency of the picking parameters. Finally, U.S. Pat. No. 6,016,287 (Klebba and van Bemmel, 2000) provides a method for relocating manual picks to depths where they would have been picked automatically.